The term "mesophase dope" used herein is defined by F. C. Frank, Discussin Faraday Society, 25, 19(1958), as follows. That is, the term "mesophase dope" refers to a dope, which exhibits such a property that the movement of the centers of gravity of the molecules, from which the dope is formed, is carried out fluidly, and the change in orientation of the molecules is carried out elastically. The mesophase dope of the present invention should be distinguished from a dope having such a property that, when the dope is placed in a fluid dynamic field, for example, when a fluid dynamic force is applied to the dope so as to create a velocity gradient of the dope and, therefore, to produce a shearing force on the dope, a fluid birefringence is created in the dope. In other words, the mesophase dope of the present invention is a dope exhibiting a bright interference color visible to the naked eye due to a certain type of systematic arangement or orientation of the molecules from which the dope is formed, even in the core where no stimulation is applied from the outside to the dope. In still other words, the mesophase dope of the present invention is a dope exhibiting liquid and solid properties that the dope exhibits a bright field of view under an orthogonal Nicol's prism of a polarizing microscope. In this regard, the term "mesophase dope" has the same meaning as the terms "liquid crystal", "optical anisotropic liquid" and "ordered liquid". In the present invention, the mesophase dope is a liquid crystal created by an interaction between a polymer and a certain solvent. Therefore, when the mesophase dope of the present invention is subjected to a wet spinning process, the resultant filaments have a high degree of orientation of the polymer molecules therein without applying a drawing procedure as in the case of ordinary spun filaments.
It is known that solutions or melts of various polymers having rigid molecular chains, for example, synthetic polypeptides, aromatic polyamides, aromatic polyamidehydrazides, aromatic polyazomethines and aromatic polyesters, form mesophase dopes. Flory (Proceedings of the Royal Society, Series A 234,73(1956)) conducted research on a statistical treatment of rigid polymer dope and provided a general explasion of the free energy in mixing of rigid polymers, as a function of the mole number of the rigid polymer, axial ratio and disorientation coefficient of solute molecule. Also, Flory predicted a separation of an optical anisotropic phase from isotropic phase of the rigid polymer at a critical concentration of the rigid polymer dope. The phase separation is derived from asymmetry of the particles in the dope. That is, in the dope of the rigid polymer, the concentration of the anisotropic phase remarkably increased due to the relatively small positive interaction energy.
In a dope of a semi-flexible polymer the molecules of which have a certain flexibility, the mesophase property of the dope depends mainly upon the length of a rigid segment in the semi-flexible molecule.
Accordingly, it is practically and theoretically supported that a specific solution of a rigid polymer or semi-flexible polymer can form a mesophase dope.
Since Flory et al studied the polymer dopes, various discussions have been made on the flexibility of molecular chains of various cellulose derivatives. However, the discussions were not correct, because the draining effect of solvent was not properly considered in the discussions.
Recently, Kamide et al (Polymer Journal, 10, 4,409(1978)) carefully analyzed data concerning the properties of solutions of various cellulose derivatives and reached a conclusion concerning the flexibility of the molecular chains in the cellulose derivatives. Concerning the conclusion, it should be noted that the expanse of the molecular chain of the cellulose derivative, and the rigidity of the molecular chain in an unperturbation state are remarkably variable depending on the type of solvent used, and the rigidity of the molecular chain of the cellulose derivative is definitely higher than that of vinyl polymers. The above-mentioned properties of the cellulose derivative are derived from the polar hydroxy (-OH) groups in its molecule and the hetero-oxygen atom located between molecules, and therefore, are variable depending upon the degree of substitution of the cellulose molecule.
Japanese patent application Laid-open (kokai) No. 52-96230 discloses the fact that an optical anisotropic dope is obtained from a combination of a cellulose derivative having a degree of substitution of 1.0 or more with a specific solvent. From the teaching of the cellulose chemistry, it is known that an increase in the degree of substitution of hydrogen atoms in the hydroxy groups in cellulose molecules by substituents, especially, hydrophobic substituents, for example, alkyl or ester groups, causes the solubility of the resultant cellulose derivative in an organic solvent to be increased. However, generally, it is difficult to obtain a uniform solution by dissolving the cellulose derivative in an organic solvent, because of formation of partial gel in the solution. In the case of a mesophase dope in which a cellulose derivative has to be dissolved in a very high concentration of 15% by weight, or more in the organic solvent, the formation of the partial gel is promoted. Therefore, in this case, it is very difficult to obtain a uniform structure of mesophase dope of the cellulose derivative. Also, the non-uniform dope can not be convented into a shaped article having a uniform quality. Furthermore, it is very difficult to completely eliminate the organic solvent from the resultant shaped article. This last difficulty causes a problem in the quality of the resultant shaped article.
The above-mentioned Japanese laid-open specification disclose several inorganic solvents for forming the mesophase dope of the cellulose derivative, however, the aqueous solution of inorganic solvent is not described as a solvent for the mesophase dope in the early publication. For example, combinations of hydroxypropylcellulose (HPC) with water, a sodium salt of carboxymethylcellulose (CMC-Na) with water, CMC-Na with an aqueous solution of sodium hydroxide, CMC-Na with an aqueous solution of sodium chloride and a sodium salt of cellulose sulfate with water, are described in the laid-open specification. However, in order to form the mesophase dopes from the above-mentioned combinations, it is necessary that the almost all of the cellulose derivatives be used in a high concentration of 50% by dry weight or more. Such high concentration of the mesophase dope is not suitable for producing shaped articles therefrom. Even in the case where a mesophase dope can be produced from about 30% by dry weight of the cellulose derivative and the inorganic solvent, the resultant mesophase dope is in the state of a paste and exhibits a poor filament-forming property. Also, the use of the above-mentioned salt solution or alkali solution causes such a problem that metal element from the inorganic solvent is retained in the resultant shaped material or that the waste water from the shaping process has to be clarified so as to avoid public polution of rivers, the sea or lakes.
On the other hand, various inorganic acids which are not described as a solvent for the cellulose derivative in above-mentioned Japanese laid-open specification, are utilized for depolymerizing cellulose materials so as to produce pulp having a desired degree of polymerization. However, due to their high depolymerizing effect, the inorganic acids have not been used as a solvent for the cellulose derivative. For example, in the preparation of cellulose acetate or cellulose nitrate, it has often been experienced in the cellulose industry that the cellulose material is remarkably depolymerized by inorganic acid, such as sulfuric acid, and the resultant product contains a certain amount of the SO.sub.4.sup.2- ion, which results in undesirable formation of gels in the solution of the cellulose derivative. From the above-mentioned experience, the use of the inorganic acid has been avoided by the cellulose industry.
Also, E. Otto and Spurline's edited Cellulose, Parts I to III, and Cellulose and Cellulose derivatives, Parts IV and V, Inter Science, in which cellulose chemistry is described in detail, contains substantially no description concerning the solubility of the cellulose derivatives in the inorganic acids, whereas the solubility of the cellulose derivatives in water or various alkali solutions or organic solvents is described very much in detail therein.
The inventors of the present invention conducted detailed studies regarding the expanse and, in its turn, the rigidity of the molecular chains of the cellulose derivatives in various solvents, in accordance with the theorem that the expanse of the molecular chains of a polar polymer in unperturbation state is promoted in a polar solvent. Also, while considering the disadvantages of the organic solvents and inorganic salt aqueous solution used as a solvent for the cellulose derivatives, the inventors of the present invention studied the production of a mesophase dope from cellulose or cellulose derivatives, for example, cellulose ethers or cellulose esters, and a specific solvent. As a result of these studies, the inventors of the present invention surprisingly discovered that a mesophase dope can be prepared from a cellulose derivative material dissolved in a solvent consisting of an aqueous solution of an inorganic acid, and the resultant dope is extremely stable in a wide range of concentration of the cellulose derivative material and in a wide range of the concentration of the inorganic acid in the dope.